Abstract Phosphatidic acid phosphatase (PAP) catalyzes the committed step in the biosynthesis of triacylglycerol (TAG) which is the dephosphorylation of phosphatidic acid (PA) to yield diacylglycerol (DAG). By the nature of its reaction PAP controls the cellular levels of PA and TAG. In Saccharomyces cerevisiae, deletion of the PAH1-encoded PAP enzyme (i.e. Scpah1? mutant) causes a 90 % reduction in cellular TAG levels. Also, the accumulation of PA in the Scpah1? causes the derepression of the phospholipid biosynthetic genes which results in increased phospholipid amounts and is manifested by the expansion of the nuclear/ER membrane. Deregulation of the human PAP enzymes, lipins, is associated with several diseases, like obesity and lipodystrophy. In this competitive grant application, we will identify the genes that encode for PAP activity in Y. lipolytica and examine their role in lipid metabolism (specific aim 1). Y. lipolytica cells exposed to high glucose amounts enter a lipogenic phase where TAG biosynthesis is induced. This results in the accumulation of TAG in the cells which can become as high as 90% of the total lipids. We will examine the role and regulation of the Y. lipolytica PAP enzymes in this metabolism (specific aim 2).